1. Field of the Invention
This invention relates to improvements in investment casting and in particular to a new and improved process for making alumina cores for employment therewith.
2. Description of the Prior Act
The production of directionally solidified (DS) metal eutectic alloys and superalloys for high pressure turbine (HPT) airfoils with intricate internal passageways for air cooling requires that the core and mold not only be dimensionally stable and sufficiently strong to contain and shape the casting but also be sufficiently weak to prevent mechanical rupture (hot cracking) of the casting during solidification and cooling. The DS process requirements of up to 1875.degree. C. for a 16 hr. time period imposes severe constraints on materials which may serve as mold or core candidates.
The prior art appears to be mostly limited to the use of silica or silica-zircon core and mold materials. At temperatures greater than 1600.degree. C. the silica based materials fail from the standpoint of both mechanical integrity and chemical incompatibility with the advanced alloy compositions.
Dimensional control of the silica core is excellent since cristobalite exhibits very little densification. Microstructural examination reveals that, in some cases, commercial core compositions employ very large particles (&gt;100 .mu.m). The addition of large particles serves to lower both shrinkage and mechanical strength.
Paul S. Svec in "Process for Making an Investment Mold for Casting and Solidification of Superalloys Therein", Ser. No. 590,970, teaches the use of alumina-silica compositions for making molds and cores. Charles C. Greskovich and Michael F. X. Gigliotti, Jr. in U.S. Pat. Nos. 3,955,616 and 3,972,367 teach cores and molds of alumina-silica compositions which have a barrier layer of alumina formed at the mold/metal interface. One possible means for the formation of their alumina layer is by a chemical reaction wherein carbon of the susceptor chemically reduces the material composition of the mold or core. Charles D. Greskovich in U.S. Pat. No. 4,046,344 also teaches an alumina-silica composition wherein the material is of a predetermined size so as to favor, and therefore enable, the formation of metastable mullite for molds and cores which exhibit superior sag resistance at high temperature.
Aluminum oxide by itself, without a chemical or physical binder material, has been identified as a potential core and mold material based on both chemical compatibility and leachability considerations. There is, however, a considerable thermal expansion mismatch between the ceramic and the alloy which generates, around the ceramic core, hoop and longitudinal tensile stresses in the alloy on cooling from the DS temperature. The high elastic modulus and high resistance to deformation at elevated temperatures of dense alumina and its lower coefficient of thermal expansion than the alloy result in the mechanical rupture or hot tearing of the alloy.
A mechanism by which an alumina core body can deform under the strain induced by the cooling alloy has been developed to permit the production of sound castings. The microstructure of the ceramic core and mold has been tailored to permit deformation under isostatic compression at a stress low enough to prevent hot tearing or cracking of the alloy. The surface of the core and mold also serves as a barrier to metal penetration. The core has a central portion having continuous porosity therein and good crushability characteristics. A barrier layer having discontinuous porosity encompasses, and is integral with, the central portion of the core. The core is easily removed from the casting with minimum effort and little effect on the quality of the cast metal by autoclave aqueous caustic solutions.
However, it has been discovered in cores having a complex geometry that the dimensional tolerances desired are difficult to attain. In fact, nonuniform shrinkage often occurs which results in minor but unacceptable distortions. The distortion results from the oxidation of carbon, a fugitive filler material, from the core in a non-uniform fashion. Regions of a core with a high surface area-to-volume ratio lose more carbon than regions with a low surface area-to-volume ratio. Greater densification occurs in the carbon depleted regions, thus giving rise to the differential shrinkage that causes distortion of the core.
An object of this invention is to provide a new and improved core for casting directionally solidified eutectic and superalloy materials having superior porosity and crushability characteristics than prior art cores.
Another object of this invention is to provide a new and improved core for casting directionally solidified eutectic and superalloy materials, wherein the material of a central portion thereof has a continuous porous microstructure and the grain morphology is characteristic of grains which have undergone vapor phase transport action. A layer of alumina encompasses, and is integral with, the central portion. The integral outer layer has a density gradient therein and continuous porosity therein and the porosity therein is small enough to prevent metal penetration.
A further object of this invention is to provide a new and improved process for making alumina cores for use in casting superalloy materials.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.